In medicine, saline (also saline solution) is a general phrase referring to a sterile solution of sodium chloride (NaCl, more commonly known as table salt) in water, but is only sterile when it is to be placed parenterally (such as intravenously); otherwise, a saline solution is a salt water solution. The sterile solution is typically used for intravenous infusion, rinsing contact lenses, nasal irrigation, and cleaning a new piercing. Saline solutions are available in various formulations for different purposes. Salines are also used in cell biology, molecular biology, and biochemistry experiments.
Normal saline (NS or N/S) is the commonly used phrase for a solution of 0.90% w/v of NaCl, about 300 mOsm/L or 9.0 g per liter. Less commonly, this solution is referred to as physiological saline or isotonic saline, neither of which is technically accurate. NS is used frequently in intravenous drips (IVs) for patients who cannot take fluids orally and have developed or are in danger of developing dehydration or hypovolemia. NS is also used for aseptic purpose. NS is typically the first fluid used when hypovolemia is severe enough to threaten the adequacy of blood circulation, and has long been believed to be the safest fluid to give quickly in large volumes. However, it is now known that rapid infusion of NS can cause metabolic acidosis.
The solution is 9 grams of sodium chloride (NaCl) dissolved on water, to a total volume of 1000 ml. The mass of 1 millilitre of normal saline is 1.0046 gram at 22 °C. The molecular weight of sodium chloride is approximately 58.5 grams per mole, so 58.5 grams of sodium chloride equals 1 mole. Since normal saline contains 9 grams of NaCl, the concentration is 9 grams per liter divided by 58.5 grams per mole, or 0.154 mole per liter. Since NaCl dissociates into two ions – sodium and chloride – 1 molar NaCl is 2 osmolar. Thus, NS contains 154 mEq/L of Na+ and Cl−. It has a slightly higher degree of osmolarity (i.e. more solute per litre) than blood (However, if you take into account the osmotic coefficient, a correction for non-ideal solutions, then the saline solution is much closer to isotonic. Osmotic coefficient of NaCl is about 0.93; therefore 0.154 × 1000 × 2 × .93 = 286.44) Nonetheless, the osmolarity of normal saline is a close approximation to the osmolarity of NaCl in blood.
One litre of 0.9% Saline contains:
For medical purposes, saline is often used to flush wounds and skin abrasions. Normal saline will not burn or sting when applied.
Saline is also used in I.V. therapy, intravenously supplying extra water to rehydrate patients or supplying the daily water and salt needs ("maintenance" needs) of a patient who is unable to take them by mouth. Because infusing a solution of low osmolality can cause problems, intravenous solutions with reduced saline concentrations typically have dextrose (glucose) added to maintain a safe osmolality while providing less sodium chloride. As the molecular weight (MW) of dextrose is greater, this has the same osmolality as normal saline despite having less sodium.
Saline was also used for the electric chair. Before the headpiece was strapped down to the victim, a small sponge soaked in the contents would be placed on the victim's head.
Saline is also often used for nasal washes to relieve some of the symptoms of the common cold. The solution exerts a softening and loosening influence on the mucus to make it easier to wash out and clear the nasal passages for both babies and adults. In this case "home-made" saline may be used: this is made by dissolving approximately half a teaspoon of table salt into 240ml (approx. 8 ounces) of clean tap water. In very rare instances, amoeba Naegleria fowleri infection can occur if amoeba enters the body through the nose, therefore water used for nasal irrigation should be sterile.
Ophthalmic drug administration
Eye drops are saline-containing drops used as an ocular route to administer. Depending on the condition being treated, they may contain steroids, antihistamines, sympathomimetics, beta receptor blockers, parasympathomimetics, parasympatholytics, prostaglandins, non-steroidal anti-inflammatory drugs (NSAIDs) or topical anesthetics. Eye drops sometimes do not have medications in them and are only lubricating and tear-replacing solutions.
Injection/ophthalmic drug administration
Syringe designed saline drops (e.g. Wallace Cameron Ultra Saline Minipod) are distributed in modern needle-exchange programmes as drugs efficiently can be administrated either by injection, or ophthalmic, which is compared to intravenous use; By demonstration, the elimination of latanoprost acid from plasma is rapid (half-life 17 minutes) after either ophthalmic or intravenous administration. However, ophthalmic use is done with sterile filtered drugs that is potent in quite small doses, and with adjusted acidity of pH 7.0-7.5 after the drug has been added, to avoid eye damage. The human eye has a pH of approximately 7.5, water has 7.0.
Hypertonic saline (NS) — 7% NaCl solutions are considered mucoactive agents and thus are used to hydrate thick secretions (mucus) in order to make it easier to cough up and out (expectorate). 3% hypertonic saline solutions are also used in critical care settings, acutely increased intracranial pressure, or severe hyponatremia. Inhalation of hypertonic saline has also been shown to help in other respiratory problems, specifically bronchiolitis. Hypertonic saline is currently recommended by the Cystic Fibrosis Foundation as a primary part of a cystic fibrosis treatment regimen.
An 11% solution of xylitol with 0.65% saline stimulates the washing of the nasopharynx and has an effect on the nasal pathogenic bacteria. This has been used in complementary and alternative medicine, and anecdotal evidence shows possible benefit for preventing infectious problems
Mechanism of action
Aerosol — Nebulized hypertonic saline treatments disrupt the interaction between glycosaminoglycans and IL-8, rendering IL-8 susceptible to proteolytic degradation with subsequent decrease in neutrophil chemotaxis; all of this ultimately reducing inflammation.
Other concentrations commonly used include:
- Half-normal saline (0.45% NaCl), often with "D5" (5% dextrose), contains 77 mEq/L of Na and Cl and 50 g/L dextrose.
- Quarter-normal saline (0.22% NaCl) has 39 mEq/L of Na and Cl and almost always contains 5% dextrose for osmolality reasons. It can be used alone in Neonatal Intensive Care Units.
- Hypertonic saline may be used in perioperative fluid management protocols to reduce excessive intravenous fluid infusions and lessen pulmonary complications. Hypertonic saline is used in treating hyponatremia and cerebral edema Rapid correction of hyponatremia via hypertonic saline, or via any saline infusion > 40 mmol/L (Na+ having a valence of 1, 40 mmol/L = 40 mEq/L) greatly increases risk of central pontine myelinolysis (CPM), and so requires constant monitoring of patient response. Water privation combined with diuretic block does not produce as much risk of CPM as saline administration does; however, it does not correct hyponatremia as rapidly as administration of hypertonic saline does. Due to hypertonicity, administration may result in phlebitis and tissue necrosis. As such, concentrations greater than 3% NaCl should normally be administered via a central venous catheter, also known as a 'central line'. Such hypertonic saline is normally available in two strengths, the former of which is more commonly administered:
- 3% NaCl has 513 mEq/L of Na and Cl.
- 5% NaCl has 856 mEq/L of Na and Cl.
- NaCl solutions that are less commonly used are 7% (1200 mEq/L) and 23.4% (approx 4000 mEq/L), both of which are used (also via central line), often in conjunction with supplementary diuretics, in the treatment of traumatic brain injury.
- Dextrose (glucose) 4% in 0.18% saline is used sometimes for maintenance replacement.
Solutions of saline with added ingredients
In medicine, common types of salines include:
And in cell biology, in addition to the above the following are used:
- Phosphate buffered saline (PBS) (recipes from Dulbecco = D-PBS, Galfre, Kuchler, Ausubel etc.)
- TRIS-buffered saline (TBS) (recipes from Goldsmith, Ausubel etc.)
- Hank's balanced salt solution (HBSS)
- Earle's balanced salt solution (EBSS)
- Standard saline citrate (SSC)
- HEPES-buffered saline (HBS) (recipes from Dittmar, Liu, Ausubel etc.)
- Gey's balanced salt solution (GBSS)
Saline was believed to have originated during the Indian Blue Cholera pandemic that swept across Europe in 1831. William Brooke O'Shaughnessy, a recent graduate of Edinburgh Medical School, proposed in an article to medical journal The Lancet to inject cholera patients with highly oxygenated salts to treat the "universal stagnation of the venous system and rapid cessation of arterialisation of the blood" seen in severely dehydrated cholera patients. He found his treatment harmless in dogs, and his proposal was soon adopted by the physician Thomas Latta in treating cholera patients to beneficial effect. In the following decades, variations and alternatives to Latta's solution were tested and used in treating cholera patients. These solutions contained a range of concentrations of sodium, chloride, potassium, carbonate, phosphate, and hydroxide. The breakthrough in achieving physiological concentrations was accomplished by Ringer in 1831, when he determined the optimal salt concentrations to maintain the contractility of frog heart muscle tissue. Normal saline is considered a descendant of the pre-Ringer solutions, as Ringer's findings were not adopted and widely used until decades later. The term "normal saline" itself appears to have little historical basis, except for Hartog Jakob Hamburger's 1882–83 in vitro studies of red cell lysis that incorrectly suggested that 0.9% was the concentration of salt in human blood (rather than 0.6%, the true concentration).
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